Naval Anti-Aircraft Guns and Gunnery. Norman Friedman

      Between the wars the Royal Navy became interested in using all of its guns to beat off air attacks. That meant setting up procedures to use heavy guns (normally limited to LA fire) to create barrages through which attacking aircraft would have to fly. The barrage concept explains why early British heavy cruiser guns elevated to 70° and why the 6in secondaries shown on Nelson in 1938 could elevate to 60°. They were by no means anti-aircraft guns: they loaded at a fixed elevation (5°), hence had to depress and elevate between shots. A project to link them with the ship’s HA control system was abandoned. The Japanese followed much the same reasoning in providing some of their 5in destroyer guns with high elevation (up to 75°). Like the British, they did not provide special anti-aircraft fire controls, and the guns fired slowly at high angles because they loaded at low elevation. Nelson’s anti-aircraft guns (single 4.7in and octuple pom-poms) are all shrouded.

      As of 1933, policy was for the modern cruisers to use their main batteries against aircraft at short ranges (this did not extend to the older ‘C’, ‘D’ and ‘E’ class cruisers). After further Mediterranean Fleet trials, a provisional method of barrage control was issued to the fleet. By 1935 barrage fire was being considered as an anti-dive bomber measure.

      In the spring of 1933 trials on board Nelson showed that her 6in guns could be used against aircraft, but with so much remaining to be done to make the specialised high-angle armament effective (according to Progress in Naval Gunnery for 1936), it seemed unwise to provide elaborate high-angle controls for low-angle guns. In 1935 the question was reopened; geometric data might be provided by a ship’s HACS. CinC Mediterranean Fleet, who was facing the threat of Italian air attacks in the Abyssinian Crisis, suggested that there would be occasions when it would be desirable to augment fleet anti-aircraft fire. That would apply more strongly to destroyers with all-low-angle batteries. He wanted barrage fire using pre-set fuses. When the rangefinder indicated that aircraft were in range, guns would open rapid fire for 30 seconds. Lateral deflection would be estimated in advance, and vertical deflection included in the guns’ elevation. This technique would produce a series of bursts on the line of sight at different heights, rather than a barrage at one fixed point in the sky.

      The following year Progress in Naval Gunnery reported increased interest in close-range barrage fire, as theoretical investigation showed that it offered a good chance of inflicting damage. Nelson, Rodney and Achilles (Home Fleet) were ordered to investigate this kind of fire during their 1936 Summer Cruise. It turned out that the most promising means to applying deflection was an HADFAS worked by the HA control officer. Its movements worked pointers on the HACS deflection screen. A prototype was fitted to Coventry, and this device was incorporated in the design of HACS Mk IV. The exercises showed that fire tended to be opened late, so it had to be ordered when even a relatively slow target was about 1000 yds beyond the barrage range setting.

      Barrage practice was further formalised in 1937. There were now three types: distant, to harass aircraft when they were first sighted; high, to harass aircraft just before they reached the point of release; and close: against aircraft carrying out close-range attacks. It was reiterated that battleship main batteries were not to be used. The secondary batteries of the Nelsons could be used for all three functions; other battleship secondaries were usable only for close-in; modern cruisers could do all three, but the ‘D’ and ‘E’ classes were limited to close-in. ‘D’ and earlier class destroyers were limited to close-in, but later ones could be used for distant and close barrages. The carriers Eagle and Furious were limited to close-range barrage. Control would be extemporised, with close-in barrage the main priority. Anything else had to give way to the urgent need to modernise fleet anti-aircraft firepower and to equip new ships. Tactics were developed.⁴⁴

      In 1938 Nelson reported a barrage technique using her 6in guns against bombers; it was estimated that fifty rounds of 6in HE were equivalent to 250 to 300 rounds of 4.7in high-angle fire. The ship’s technique was reported to the fleet.⁴⁵ The idea was to fire a full broadside at a fixed fuse setting, ‘rippling’ fire to increase the period of time during which shell was bursting in the selected zone. The technique would be used only against approaching targets, shells being fused to burst on the line of sight. The recommended rippling interval for 6in shells was 2 seconds. Four fixed fuse-settings were specified for particular ranges and angles of sight. By this time the use of low-angle guns for air defence was considered so important that a special computer was being developed specifically to use HACS data for low-angle fire control.

      Close-range barrage fire was now recognised as an important part of fleet air defence, to be imposed both by long-range anti-aircraft guns and by low-angle guns. Adding heavy anti-aircraft was a new departure. To some extent it was an admission that HACS was limited, and it was also a way of supporting a high rate of fire, which was essential if no gaps were left through which a fast target could pass. It was described as a way of filling the gap between the shortest range at which controlled fire was effective and the range at which a bomb or torpedo would be dropped. Against a torpedo bomber the preferred setting was 1500 yds. The attacker would be forced either to drop the torpedo at excessive range or to pass through the barrage. In either case there was no point in a shorter-range barrage; by this time it was clear that barrage range should not be changed by any step less than 1000 yds. Barrage was also a way to deal with dive bombers, 1000 yds being the most suitable range. The provision of forward area sights was connected with barrage tactics.

      Barrage fire by anti-ship guns offered the ability to engage a separate set of targets during a synchronised attack. During the Abyssinian Crisis the idea of a long-range barrage by anti-ship guns became so important that it overshadowed the close-range barrage (which had been far more important). As of 1938 control methods were still being developed.

      Detailed instructions for barrage fire were issued in the spring of 1939, with special emphasis on dive bombers. Controlled fire would be continued as long as possible, but once bombers broke formation to dive, the HACS could no longer control fire, and a barrage had to be opened. The problem of torpedo attack also seemed to invite a barrage solution.

      Detection

      The committee was also interested in the evolving science of aircraft detection by sound. After witnessing an October 1919 demonstration of the army’s system, the committee suggested rigging one aboard a ship. The army system consisted of a large aimable disc to focus sound, tuned to a frequency corresponding to an aircraft exhaust. In May 1920 the Committee laid out specifications for a shipboard sound locator, to be scanned over the sky. It should indicate the presence, direction, and altitude of an aircraft or airship up to 15 miles away, with an accuracy of at least 5°, neglecting distortion due to wind, etc. The device should be insensitive to non-aircraft sounds. Some form of IFF would be desirable. The project soon collapsed; later naval interest in sound detection concentrated on the problem of defending the fleet in harbour.

      It was obvious that lookouts were not enough, particularly as aircraft became faster and flew higher. That was one reason radar was so important. The first British naval radars, beginning with Type 79, were air-warning sets. Because they operated at relatively low frequency (the best technology available in quantity), shipboard antennas could not offer very fine beam definition. However, they could produce enough power to detect aircraft at a considerable distance. These sets were classified as aircraft warning (AW) devices. By 1940 they had been refined to the point where they provided good ranges. They could not support director fire control, but they could support barrage fire, because they could indicate that an incoming target was passing within barrage range.

      By the outbreak of war work was well advanced on a second generation of radars operating at shorter wavelength (about 50cm compared to 3.5m or 1.5m), offering ranging but not precise direction for fire control: Type